A power save mode of operation for wireless communications devices, such as a mobile phone, is primarily employed to preserve battery life during the time that these devices are turned on but not transmitting. If a mobile phone has been configured such that the power save mode is active, it will periodically transition from a higher powered state, in which the devices transceiver is turned on, to a lower powered state, in which the device's transceiver is turned off, during periods of inactivity. In this lower powered state, the mobile phone will periodically “wake up” or transition to a higher powered state for a short time in order to receive management information from the access point with which it is associated and then transition back to the lower powered state. Also, the phone will transition to a higher powered state in the event that the user wishes to send a voice message. Generally, an access point will buffer one or more frames of information addressed to a particular mobile phone that is power save mode enabled and which is in a lower powered state until the phone transitions to a higher powered state, at which time the buffer frames could be transmitted to the mobile phone. The process by which a mobile phone is configured to be power save mode capable and the protocol that a wireless LAN (access point) follows to manage the buffering and transmission of frames of information is specified by the IEEE 802.11 standard.
While the power save capability specified by 802.11 addresses the battery life issue, it does not address quality of service issues. To this end, the IEEE 802.11e standard which extends 802.11 into the area of quality of service was developed. Generally, the 802.11e standard specifies formats and protocols to use at an access point and at a mobile phone that enable a wireless LAN to prioritize the delivery of packets of information according to the user priority or access category the packets are assigned. Specifically with respect to 802.11e there are two different power save modes that have been specified by the standard for use to deliver packets of information that have been labeled for priority delivery. One mode allows for the scheduled delivery of traffic by the AP to a phone during scheduled service periods and is referred to as Scheduled Automatic Power Save Delivery (S-APSD) and a second allows for the unscheduled delivery of traffic by the AP to a phone during unscheduled service periods and is referred to as Unscheduled Automatic Power Save Delivery or U-APSD. These scheduled and unscheduled modes of operation are defined from the perspective of the AP and should be used when it is desirable to provide QoS functionality in conjunction with extended battery life. It is recommended that the S-APSD mode be employed by devices using polled access and that the U-APSD mode be employed by devices using contention-based access. Basically, APSD was developed to add QoS functionality to the standard PS-mode.
An unscheduled service period begins when an access point receives a trigger frame from a phone. In response to receiving a trigger frame from a particular mobile phone, an access point will transmit any packets it has buffered to that mobile phone. Trigger frames transmitted by phones when in U-APSD mode can collide with trigger frames transmitted by other phones associated with the same access point causing one or more of the phones to initiate a back-off procedure which has the effect of adding delay to the transmission of voice information. The effect of “collisions” between trigger frames tends to counteract the benefit of U-APSD QoS functionality. Further, U-APSD makes no allowance for prioritizing the delivery of packets, buffered at an access point for delivery to some number of different phones, to any one phone associated with the access point, but rather only prioritizes the buffered packets for delivery to a particular phone.
Therefore, it would be advantageous if a phone could periodically transmit a trigger frame without the possibility that this trigger frame might collide with any other phones trigger frame. Furthermore, it would be beneficial if one phone had access to the medium on a priority basis over other phones also trying to acquire the medium to send voice or data traffic. Still further, it would be beneficial if phones did not have to maintain long lists of phones associated with the same access point.
We have solved the trigger frame collision problem and we have solved the packet prioritization problem between phones while operating in the U-APSD mode by inventing a method that enables a phone to start an unscheduled service period immediately after the end of another phones unscheduled service period without having to contend for the medium in the normal fashion. Further, the method of our invention allows the service periods of all phones associated with a particular access point to be synchronized such that the trigger frames they transmit will not collide with trigger frames transmitted by the other phones associated with the same access point. Still further, the method of our invention arranges the service periods of multiple phones to be consecutively ordered one after the other and separated in time by a minimum inter frame time duration also resulting in more time being available during an unscheduled service period to conduct such operations a dynamic channel access.